Mass spectrometry (MS) requires sample molecules to be converted into ions before mass analysis can be performed. There are a variety of ionization techniques used in MS, among which the most common include electron impact (EI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photo ionization (APPI), matrix-assisted laser desorption/ionization (MALDI) and electro-spray ionization (ESI).
EI only works for gas and volatile organic molecules, wherein thermally excited electrons interact with gas phase atoms or molecules to produce ions, and is coupled predominantly with gas chromatography (GC). EI, due to its high degree of fragmentation, is not suitable for bio-molecules, which tend to be fragile and easily fragmented. APCI works for both gas and liquid, in which ions are produced through the collision of sample molecules with ions of a reagent gas. APCI requires high temperatures, typically above 400° C., to convert a liquid sample into a gaseous state before the sample can be ionized by chemical ionization initiated by a corona discharge electrode. Hence, APCI, like EI, is not well suited for bio-molecules. APPI is similar to APCI except the chemical ionization is initiated by photons.
MALDI and ESI are soft ionization techniques and can be applied to a wide range of samples, including fragile bio-molecules, e.g., amino acids, peptides and proteins. MALDI entails samples dried on a surface so is not well suited as an online MS interface, e.g., for online ionization of a sample eluted from a liquid chromatography (LC) column. As a result, MALDI is normally not coupled with an online separation technique. ESI works for liquid samples and is regarded as an ideal interface to couple a LC module directly online to a mass spectrometer. Nonetheless, ESI requires a high voltage, typically in a range of 2 kV to 6 kV, to promote ionization and such a requirement is not only expensive but also hazardous. And both MALDI and ESI have difficulty with non-polar samples.
One technique for ionizing samples without application of a high voltage or high temperatures entails directing a jet, formed by emitting liquid carbon dioxide from an orifice, to a surface containing samples. Though such a technique involves no voltage nor high temperatures, it requires samples to be on or in a surface, and therefore is not well suited for directly coupling a LC module to a mass spectrometer.